1. Field of the Invention
The present invention relates to a method for passivating the interior of reactors which are subject to coking, such as hydrocarbon pyrolysis tubes, in order to reduce the rate of coke formation, in particular during the production of ethylene. The invention relates both to conventional ethylene production which uses steam in the feedstock as a diluent and to nonconventional production which employs steamless cracking of feedstock. The present invention also relates to an improved apparatus exhibiting a reduced tendency to undergo coking produced by such a method and a method for pyrolyzing hydrocarbons utilizing such an apparatus.
2. Discussion of the Background
Coking is a significant problem in high-temperature chemical reactions, such as the pyrolysis of hydrocarbons, particularly in the production of ethylene.
Ethylene, the lightest olefinic hydrocarbon, is the most important building block of the petrochemical industry. Ethylene is produced almost exclusively via the pyrolysis of hydrocarbons in tubular reactor coils which are externally heated by a furnace (see: Chapter 8 in Pyrolysis of Hydrocarbons, p.109-142, Marcel Dekker Inc., New York (1980)). High selectivity toward the production of desired olefins (i.e., ethylene and propylene) and diolefins (i.e., butadiene) with minimum methane and hydrogen production and coking in the coils leading to longer heater runs are desired. This is achieved by operating the pyrolysis heaters at high temperatures (750-900.degree. C.), short residence times (0.1-0.6 sec.) and low hydrocarbon partial pressures. Steam is added to the feedstock to reduce the hydrocarbon partial pressure and the amount of carbon deposited on the tube walls.
Steamless cracking has been investigated as a potential means of increasing production capacity and maximizing energy efficiencies (see "Steamless Pyrolysis of Ethane to Ethylene", Paper 101, presented at a meeting of the American Chemical Society, Boston, Mass., April 1990, by Y. Song, A. A. Leff, W. R. Kliewer and J. E. Metcalf). The work cited above was performed in a tube made entirely of silicon carbide. The use of tubes constructed of silicon carbide, however, would not be possible on an industrial scale because of the low mechanical reliability and fabrication problems of this material.
Tubular reactor coils, also known as pyrolysis heaters, are an important facet of operation to consider partly because of coke deposition (see: Kirk-Qthmer Encyclopedia of Chemical Technology, Vol. 9, "Ethylene", J. Wiley & Sons Inc., New York (1979)). The mechanism of coke formation has been postulated (see L. F. Albright & J. C. Marck, Ind. Eng. Chem. Res., vol 27, 731 and 743 (1988)), but has yet to be modeled in precise terms.
The reduction of the coking rate and the extension of the reactor run time have been the subject of several investigations and commercial applications (see for example the Products Bulletins G-262, G-5263, G-5267, G-5268 by Nalco Chem. Co., Petroleum and Process Chemical Division, 1800 Eperson Bldn.--Houston, Tex.).
For instance, the use of a silicon dioxide layer to inhibit coke formation inside thermal cracking reactors is known from UK-1,332,569 and U.S. Pat. No. 4,099,990. In particular, in U.S. Pat. No. 4,099,990, the silicon dioxide coating is obtained by thermal decomposition of an alkoxysilane in the vapor phase. The silicon dioxide coating reduces coking rates. Although any non-catalytic surface would be effective for coke reduction the factors which determine industrial applicability of a coating material are the following: the thermal expansion match of the layer with the metal, the melting point of the coating material, the degree of strength, brittleness, adherence, the resistance to wear and corrosion, and so on. From this point of view, silicon dioxide films suffer from many drawbacks, mainly due to the wide gap between the thermal expansion coefficients of silicon dioxide and of the metal substrate. This mismatch causes poor adhesion of the layer to the substrate, poor thermal shock and spallation resistance.
U.S. Pat. No. 3,536,776 discloses a method of reducing coke in the high temperature conversion of hydrocarbons by utilizing a reactor which is coated with a metal ceramic material containing particles of a catalytically inert, refractory solid ceramic substance dispersed in chromium.
Thus, there remains a need for an apparatus which exhibits a reduced tendency to undergo coking. In particular, there remains a need for a method and an apparatus for pyrolyzing hydrocarbons which are free of the above-described drawbacks. There also remains a need for a method for producing such an apparatus.